The Renovation Program - Ðổi Mới in Vietnam since 1986 have posed a puzzling policy question: why have some policy areas experienced radical changes while others have experienced only limited and incremental changes? This policy puzzle provided the focus for this dissertation in which a model of major policy change was developed to provide a new way of explaining the policy reforms in Vietnam over the past two decades. The model was developed based on three bodies of literature: (1) the most well-developed theories and models of policy change process created in the U.S and their application to the non-U.S policy contexts; (2) the Policy-elite model as an alternative to explain the policy reforms in developing countries; (3) critical and unique regime characteristics of Vietnam that play an important role in shaping the policy contexts for the policy processes and outcomes in Vietnam. Taken together, these bodies of literature provided the basic concepts and suggested potential causal mechanism of major policy change for a conceptual framework to build a major policy change model for Vietnam. The proposed policy model identifies four policy factors (stressor, leadership predisposition, change in policy image and consensus on the political priority) that need to occur at different stages of the policy process in Vietnam to make radical change happen. Owning to the unique regime characteristics of Vietnam, the model differs from other policy process theories and models in the way that it strongly emphasizes the role of the Communist Party and the predisposition to reform embraced by the policy elites in the process of major policy change. It also reflects the collective and consensus-based policy making style of the Vietnamese Communist Party and government in the transitional period of the country. The explanatory capacity of the proposed policy model was validated by four policy case studies in higher education, international trade liberalization, state economic sector, and legal reform in foreign investment in Vietnam. The empirical evidence drawn from the case studies has affirmed the usefulness and relevance of the policy factors and the causal flow embedded in the proposed model. Concretely, the two cases with radical policy changes witnessed the presence of all four policy factors and the processes of change followed the causal arguments of the model. Whereas, in the two cases without radical changes, the legacy of a Socialist state in Vietnam has impeded the significant changes in the policy image of the policy elites in respective policy domains. As the result, no innovative policy change alternative has been advanced to the agendas of the Vietnamese government, which in turn prohibited radical policy changes in the areas of higher education and state-owned enterprise over the past two decades. In the last chapter, the cross-case comparison has found that in all four cases, there have been strong stressors and the leaders of the Vietnamese Communist Party and government have felt great pressure to reform. The Party has shown the predisposition to reform in various guiding resolutions in the four policy sectors. Yet, in the cases of higher education policy on institutional autonomy and state-owned enterprise management policy, the lack of significant change in the policy image of the leaders has been the main reason for the absence of innovative policy change. In contrast, in the cases of international trade liberalization and legal reform in setting the level playing field for enterprises of all economic sectors, all the policy factors have occurred to produce radical policy changes in these two areas.